Current systems for measuring aspects of railroad wheels include numerous types of physical gauges, such as a “J” type steel wheel gauge approved by the Association of American Railroads (AAR) and a “pi tape” for measuring a circumference/diameter of railroad wheels. Such devices are inexpensive and simple, however, use of the device is prone to human error based on precise placement and angle of view of the devices. Furthermore, an overall accuracy is limited by human perceptual capabilities. Electronic wheel gauges also are used, including those described in U.S. Pat. Nos. 4,904,939 and 7,478,570 and a wheel diameter gauge provided by Riftek. These devices provide more accurate measurements, but are somewhat bulky and are often difficult to fit into restricted spaces afforded by transit railcar wheels.
Some devices seek to provide minimal contact or noncontact measurement. For example, a laser wheel profilometer offered by Riftek projects a single point of light from a moving carriage across a wheel, deriving a wheel profile from multiple point distance readings. The device includes moving components, which are subject to wear and breakage and often cannot fit in small areas. Structured-light based measurement of railroad wheels has been implemented in wayside and in-ground systems, such as those described in U.S. Pat. Nos. 5,636,026 and 6,768,551, both of which are hereby incorporated by reference. These measurement systems have been shown to be highly effective and reliable at measuring railroad wheels from structured light projections, but are extremely expensive, permanent installations.
Another gauge, Calipri, offered by NextSense, measures the main profile aspects of a wheel using a noncontact solution by having an operator carefully pass a sensor head around the wheel in a semicircular fashion. However, this gauge cannot measure diameter without the use of a large, separate fixture and a separate measurement. Furthermore, the gauge requires the operator to move the sensor head in a precise manner, requires a significant standoff distance, and requires a separate connected computational and display component. The gauge takes measurements from a succession of images acquired over a period of time, which must be referenced to each other with an extremely high degree of precision if the resulting measurements are to be in any way accurate. As a result, the gauge also requires use of an expensive and sophisticated inertial measurement unit (IMU) to recognize and compensate for significant variations of poses of the operator's hand while taking the measurements.